#include<iostream>

using namespace std;


 struct ListNode {
      int val;
      ListNode *next;
      ListNode() : val(0), next(nullptr) {}
      ListNode(int x) : val(x), next(nullptr) {}
      ListNode(int x, ListNode *next) : val(x), next(next) {}
  };
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == nullptr || head->next == nullptr)
        {
            return head;
        }

        ListNode* tmp = swapPairs(head->next->next);
        auto ret = head->next;
        head->next->next = head;
        head->next = tmp;

        return ret;

    }
};

int main()
{
    return 0;
}